1. FIELD OF THE INVENTION
The present invention relates to the field of integrated circuits, and specifically, sensing of injected currents at the input pins of the integrated circuits.
2. ART BACKGROUND
It is well known in the art to use integrated circuits containing microcontrollers in applications such as closed-loop control and digital signal processing. Such applications are widely utilized in products such as modems, motor controls, engine controls and medical instrumentation, to name just a few. In the field of microcontrollers, it is also common to implement an on-chip analog-to-digital (A/D) converter to convert analog input signals into digital signals to be processed by the controller. However, because a microcontroller chip is quite often operating in a noisy environment, such as in an automobile input pins connected to the on-chip A/D converter frequently experience injected currents caused by sporadic voltage spikes in the proximity of the integrated circuit device.
When there is sufficient injected current present, a parasitic diode 100 at the input pin, such as the one shown in FIG. 1, becomes forward biased due to the voltage spike. As a result, input voltages to the A/D converter pins begin ramping up or down, thus affecting the analog comparison to be performed during the A/D conversion. This problem is typically avoided by connecting discrete external parts, such as Schottky diodes with their lower turn-on voltage, to the input pins to clamp the voltages at the input pins to less than one forward diode's ON voltage drop ("V.sub.D (ON)") beyond the supply voltage such that the parasitic diodes are not forward biased by the injected current. However, as the number of pins increases due to larger and larger scales of integration, more external parts are required for the pins on each chip, making this solution impractical and expensive. Also, attempts have been made to fabricate on-chip Schottky diodes in the same process. However, various semiconductor process limitations make implementing Schottky diodes on-chip an unsuitable solution. As such, an on-chip circuit to sense and clamp an injected current becomes a preferable solution in view of the process limitations confronted by the prior art. Also, because microcontrollers often must operate in environments with varying ambient temperature, an on-chip sense circuit must be able to track the threshold behavior of the parasitic diodes independent of the varying temperature to be completely effective.
As will be described in the following description, the present invention provides a circuit that senses and clamps an injected current at each of the input pins of an integrated circuit device. When sensing an injected current at the input pin, the circuit sinks the injected current to prevent the injected current from forward-biasing the parasitic diode and degrading the accuracy of the A/D converter. The sense circuit of the present invention is ideally suited for use in sensing and clamping an injected current at each of the pins as part of an integrated circuit. In addition, the sense circuit can be easily fabricated as an integral part of the semiconductor circuit which contains the microcontroller and the A/D converter. Furthermore, the sense circuit is able to track the threshold behavior of the input parasitic diode despite the temperature variations.